This application relates to the realization of P-on-N type II InAs/GaSb superlattice photodiodes, and more particularly, to polarity inversion of type II InAs/GaSb superlattice photodiodes. The present application also relates to the realization of P-on-N Type II superlattice photodiodes with a cut-off wavelength of 11 μm and the optimization of contact layers to improve the device performance.
In recent years, Type II superlattice (SL) photodiodes have experienced significant improvements in theoretical design and experimental growth. Focal Plane Array (FPA) imaging in both Mid-wavelength Infrared and Long wavelength Infrared has also experienced improvement. However, one difficulty in the fabrication of Type II superlattice FPAs is the polarity matching with the Read Out Integrated Circuit (ROIC). While most commercially available ROICs formatted for this material system are designed for P-on-N photodiodes, most Type II FPAs reported in the literature are based on an N-on-P design. This opposite polarity forces the ROIC to operate in forward bias and reduces its readability.
Most superlattice photon detectors are grown by state-of-the-art MOCVD and MBE techniques which allow a high material uniformity across a larger wafer. Higher material uniformity translates to FPAs with a higher detector array uniformity, which is very important for lower NEAT (noise equivalent temperature difference). Due to its internal detection mechanism (absorption of photons by electrons), a quantum well photon detector has very fast response time (up to 30 GHz) compared to thermal and other infrared detectors. Fast response time FPAs are highly favored by military applications such as target tracking. With quantum well structure engineering, one may tune the detection wavelength of quantum well photon detectors and, by stacking quantum well structures, one may achieve multi-band detection.
Type-II superlattice detectors (i.e. quantum well photon detectors with type-II band alignment) have shown high room temperature detectivity and such quantum well detectors can be used to build future uncooled FPAs with fast response times. So far, most uncooled FPAs are based on slow thermal detectors, such as microbolometers.
Each multi quantum well (MQW) structure has multiple quantum wells which are artificially fabricated by alternatively placing thin layers of two different, high-bandgap semiconductor materials adjacent to one another to form a stack, as known in the art. The bandgap discontinuity of the two materials creates quantized subbands in the potential wells associated with conduction bands or valence bands.
The band alignment of any heterojunction can be categorized as type-I, type-II staggered or type-II misaligned. In type-I heterojunctions, one material has lower energy for electrons and the holes and therefore both carriers are confined in that layer. In type-II heterojunctions, however, the electrons are confined in one material and the holes in the other. In the extreme case, which is called type-II misaligned, the energy of the conduction band of one material is less than the valence band of the other one.
Type-II superlattice detectors are based on interband optical transitions and hence they can operate at much higher temperatures. Moreover, theoretical calculations and experimental results show that InAs/Ga1-xInxSb type-II superlattices have a similar absorption coefficient to HgCdTe, and therefore type-II superlattice detectors with high quantum efficiencies are possible.
The special band alignment of the type-II heterojunctions provides three important features that may be used in many devices to improve the overall performance of the device.
The first feature is that a superlattice with the type-II band structure can have a lower effective bandgap than the bandgap of each layer. This is an important issue for the applications in the mid and long infrared wavelength range, since one can generate an artificial material (the superlattice) with a constant lattice parameter but different bandgaps. Very successful detectors and lasers have been implemented in the 2-15 μm wavelength range and InAs/GaInSb superlattices lattice-matched to GaSb substrates.
The second feature is the spatial separation of the electrons and holes in a type-II heterojunction. This phenomenon is a unique feature of this band alignment and is due to the separation of the electron and hole potential wells. As a result of such spatial separation, a huge internal electrical field exists in the junction without any doping or hydrostaticpressure. High performance optical modulators have been implemented based on this feature.
The third feature is the zener-type tunneling in a type-II misaligned heterojunction. Electrons can easily tunnel from the conduction band of one layer to the valence band of the other layer, since the energy of the conduction band of the former layer is less than the energy of the valence band of the later layer. Unlike a zener tunneling junction which requires heavily doped layers, no doping is necessary for such a junction. Therefore, even a semimetal layer can be implemented with very high electron and hole mobilities since the impurity and ion scattering are very low. This feature of type-II heterojunctions has been successfully used for resonant tunneling diodes (RTDs) and recently for the implementation of type-II quantum cascade lasers.